The process of VOC destruction is called biofiltration, biodegradation, bioremediation, biotransformation and/or biooxidation. It is mainly carried out by two types of microorganisms, heterotrophic bacteria and fungi. Biodegradation is an enzymatic process whereby enzymes produced by the microorganisms catalyze the breakdown of the VOC's into carbon dioxide and water. The VOC's, in particular hydrocarbons, may or may not be used as an energy source to produce additional microorganisms. The process of biodegradation whereby a co-substrate is added to provide a source of carbon and energy for bacterial growth is called cometabolism. In a biofilter the microorganisms grow on materials such as soil, compost, peat or heather, supplemented sometimes with synthetic materials, including activated carbon and polystyrene, which provides structure stability.
Subsurface contamination of ground water and soil results from the release of petroleum products and industrial chemicals due to leaking aboveground and underground storage tanks, piping, pumps, and/or dispensers. Leakage also results from human error and acts of nature. In the case of petroleum products, subsurface contamination is often encountered at retail gasoline facilities, bulk storage terminals, transfer stations, pipelines, and refineries. Due to environmental regulations, the remediation of ground water and soils has become necessary in the petroleum industry. Subsurface remediation for the most part involves three basic processes: the extraction of fluids for treatment above ground, the injection of fluids for treatment in situ, or a combination of both. Technologies available to accomplish these processes include treatment via physical, chemical or biological methods, many of which produce a vapor or air stream requiring further treatment for regulatory compliance.
One of the most common remediation technologies involves the extraction of vapors from the unsaturated or vadose zone of the subsurface. Following the subsurface release of a volatile product like gasoline, vapors accumulate in the void spaces found in soil which are not filled with water. These vapors are extracted through vertical or horizontal slotted pipes with the use of a vacuum pump or similar device. As these vapors are removed volatilization of the product is enhanced resulting in continued vapor production. This process is called soil vapor extraction or soil venting.
A similar vapor producing process involves applying a vacuum to a vertical slotted pipe extending into the water table (i.e. ground water recovery well) simultaneously with ground water pumping. This technique not only results in venting of the vadose zone but also increases the volatilization of free phase hydrocarbons (i.e. product) floating on the ground water table and increases the recovery rates of ground water and product via pumping. This process is called vacuum enhancement.
A third vacuum technology involves the simultaneous extraction of ground water, vapor, and product from numerous vertical slotted pipes extending into the water table using a high vacuum extraction pump. This technique allows total phase extraction of fluids with one pump via many extraction points and at rates significantly higher than conventional pumping methods. Extracted fluids (i.e. water and vapor) are separated at the surface for further treatment with other processes.
Due to the regulated discharge of VOC's to the atmosphere, treatment of contaminated air streams from vapor extraction is required in most regions. Several types of air treatment systems are used in subsurface remediation including carbon adsorption, thermal combustion, catalytic oxidation and biofiltration. The use of these air treatment systems depend on the types and concentrations of contaminants, the air flow rates and the availability of energy. Thermal combustion and catalytic oxidation systems require energy input to operate. Both of these systems result in the destruction of VOC's but require high concentrations of VOC's in the air stream to be economically feasible. Carbon adsorption or carbon filtration involves passing the air stream through a vessel containing activated carbon where VOC's are adsorbed and retained. Once the carbon is saturated with VOC's it can be regenerated with thermal treatment or stored as a solid waste. Biofiltration differs from the other treatment processes because it results in VOC destruction with minimal energy input, is a regenerative process, and does not result in by-products requiring further treatment or disposal.
The basic components of a biofiltration system are a bed of filter material and a piping system that distributes the VOC's into the filter, a blower or vacuum source to propel the gas through the system and pre treatment devices for humidifying the gases and, where necessary, removing particulates.
Compost, usually produced from municipal waste, wood chips, bark or leaves has generally been the basis of filter material. Open single-bed systems and open multilevel systems are inexpensive and relatively easy to maintain and have large space requirements. However, enclosed systems, usually with stacked beds are appropriate where minimum maintenance is required and where space constraints prohibit the installation of a single-bed filter. The useful life for the filter material is up to about five years. Compaction of the compost can lead to back pressure and typically the compost needs to be turned over or eventually replaced. Drying of the filter material renders it biologically inactive and requires replacement of the compost.
The type of construction and installation of a biofilter for a given application will depend primarily on the availability of space relative to the required filter volume.
It is an object of this invention to provide a bioremediation apparatus for removing volatile organic compounds (VOC's) from air streams which includes a multilevel stacked biofilter which holds biofilter media at each level but does not directly contact media in the level above or below.
It is another object of this invention to provide a biologically active media to adsorb the VOC's resulting in their biodegradation to carbon dioxide and water.
It is another object of this invention to provide a biofilter in which each level is divided into lower and upper chambers by a perforated support plate which retains media but allows the passage of air and water vapor with minimal resistance.
It is another object of this invention to provide a multilevel biofilter in which each perforated plate is reinforced to support the weight of the media.
It is another object of this invention to provide a multilevel biofilter in which there is a void space between the media levels to isolate the levels into distinct treatment zones.
It is another object of this invention to provide a biofilter which allows access to media at each level for monitoring and sampling.
It is another object of this invention to provide a bioremediation apparatus which humidifies dry air and cools hot air prior to contacting the media.
It is another object of this invention to provide a bioremediation apparatus which is operated under a slight vacuum to cause the downward flow of air and water vapor through the media.